The studies proposed herein will provide new insights into the early steps of atherogenesis. We hypothesize that as macrophages come into contact with matrix-retained LDL aggregates, rapid cholesterol ester transfer occurs. This results in an increase in macrophage membrane cholesterol levels and changes in cell morphology and migratory ability. This process may explain why macrophages within atherosclerotic lesions continue to take up retained lipoproteins and develop into nonmotile foam cells. To investigate this hypothesis, time-lapse confocal fluorescence imaging of in vitro tissue cultures will be utilized to examine macrophage morphological alterations induced during their initial contact with matrix-associated lipoproteins. Further, monitoring the transfer of a fluorescent cholesterol analog from LDL to macrophages allows direct observation of changes in plasma membrane cholesterol levels. Finally, studies will extend our observations on in vitro tissue models to in vivo examination of macrophage migratory behavior in animal models of atherosclerosis. Intravital microscopy will be used for quantification of monocyte/macrophage diapedesis. Results may eventually lead to innovative methods of promoting atherosclerotic lesion regression. [unreadable] [unreadable] [unreadable]